Light-emitting devices may be generally divided into organic light-emitting devices (OLEDs) in which a light-emitting layer is formed of an organic material and inorganic light-emitting devices in which a light-emitting layer is formed of an inorganic material. OLEDs are self-emitting light sources based on the radiative decay of excitons in an organic light-emitting layer, the excitons being generated by the recombination of electrons injected through an electron injection electrode (cathode) and holes injected through a hole injection electrode (anode). OLEDs have a range of merits, such as low-voltage driving, self-emission, a wide viewing angle, high resolution, natural color reproducibility, and rapid response rates.
Recently, research into applying OLEDs to a variety of devices, such as portable communications devices, cameras, watches, office equipment, vehicle information display devices, televisions (TVs), display devices, illumination systems, and the like, has been actively undertaken.
In order to improve the luminous efficiency of OLEDs, it is necessary to improve the luminous efficiency of a material that constitutes a light-emitting layer or to improve light extraction efficiency in terms of a level at which light generated by the light-emitting layer is extracted.
Here, light extraction efficiency depends on the refractive indices of the layers of materials that constitute an OLED. In a typical OLED, when a beam of light generated by the light-emitting layer is emitted at an angle greater than a critical angle, the beam of light may be totally reflected at the interface between a higher-refractivity layer, such as a transparent electrode layer, and a lower-refractivity layer, such as a glass substrate. This consequently lowers light extraction efficiency, thereby lowering the overall luminous efficiency of the OLED, which is problematic.
More specifically, only about 20% of light generated by an OLED is emitted out and about 80% of the light generated is lost due to a waveguide effect originating from the difference in refractive indices between a glass substrate and an organic light-emitting layer that includes an anode, a hole injection layer, a hole transport layer, an emissive layer, an electron transport layer, and an electron injection layer, as well as by the total internal reflection originating from the difference in refractive indices between the glass substrate and ambient air. Here, the refractive index of the internal organic light-emitting layer ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO), generally used for the anode, is about 1.9. Since the two layers have a significantly low thickness, ranging from 200 nm to 400 nm, and the refractive index of the glass used for the glass substrate is about 1.5, a planar waveguide is thereby formed inside the OLED. It is estimated that the percentage of light lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate is about 1.5 and the refractive index of the ambient air is 1.0, when light exits the interior of the glass substrate, a beam of the light having an angle of incidence greater than a critical angle is totally reflected and trapped inside the glass substrate. The ratio of the trapped light is commonly about 35%, and only about 20% of generated light is emitted out.
Therefore, research into methods for improving the light extraction efficiency of OLEDs is being actively undertaken.